In a typical NMR experiment, a sample is placed in a uniform static magnetic field (B.sub.0) and excited by a Rf magnetic field (B.sub.1) applied in a direction orthogonal to the static magnetic field. An NMR probe holds the sample coaxially within a resonator (or coil), which generates the Rf magnetic field. The resonator also detects the resonance signal of the sample, which is delivered to a receiver circuit to obtain the NMR spectrum.
While the sample is within the resonator, ionic species in the sample tend to electrically couple to the resonator, which causes losses in the NMR signal. To prevent electric coupling of the sample to the resonator, a Rf shield 10a, 10b is placed inside the resonator 12, between the resonator and the sample (not shown), as FIG. 1 illustrates. The Rf shield comprises two conductive rings 10a, 10b, one of which is placed coaxially at each end of the resonator, but without extending into the Rf window 14 of the resonator. Generally, the Rf shield is made of a susceptibility compensated conductive material and can be encased in a dielectric material, such as quartz, such that the conductive rings capacitively couple with the resonator.
For double resonance NMR experiments, the probe comprises two resonators, with an inner resonator placed coaxially within an outer resonator such that the Rf magnetic fields generated by the two resonators within the probe are orthogonal to each other. Most commonly, one resonator is tuned to the frequency of proton (.sup.1 H) excitation, while the other resonator is tuned to a lower resonant frequency nuclide of interest (X).
To increase the penetration of the lower frequency Rf magnetic field into the sample, axial slots are made in the other resonator. FIG. 2 illustrates a double resonance NMR probe having the slotted resonator 22 within the lower frequency coil 20. The two axial slots 24a, 24b in the slotted resonator 20 are placed 180.degree. from each other and 90.degree. out of phase with the Rf window 26 of the slotted resonator. This arrangement permits greater penetration of lower frequency Rf field through the slotted resonator into the sample.
Lengthening of the axial slots allows for greater penetration of lower frequency Rf field into the sample, and also allows the lower frequency resonator to be lengthened, which desirably increases lower frequency Rf field homogeneity. However, lengthening the axial slots in the slotted resonator is not without limit. Lengthening of the axial slots would require an accompanying lengthening of the Rf window, and in most cases, such drastic changes to the geometry of the slotted resonator would result in a change in the self-resonance frequency of the resonator and so could not be tolerated.
It is known in prior art to employ in probe components, slots of various types and orientations to accomplish reduction of eddy currents, increased transparency to RF irradiation from an outer coil, and other desired results. Representative references are patents U.S. Pat. No. 4,641,098 to Doty; U.S. Pat. No. 5,192,911 to Hill and Cummings; U.S. Pat. No. 4,875,013 to Murakami, et al; U.S. Pat. No. 4,929,881 to Yabusaki, et al; and U.S. Pat. No. 4,748,412 to Yamamoto, et al.